A DC motor can draw very high current at startup because back EMF is zero or very low at standstill. A 4-point starter helps solve this by adding resistance during starting and gradually removing it as speed increases. It provides no-volt and overload protection, and is more suitable for field-weakening speed control than a 3-point starter. This article provides information on its parts, operation, uses, limitations, and maintenance.
4-Point Starter Overview
A 4-point starter is a starting and protection device used with DC shunt and compound motors. It limits the high armature current at startup, when back EMF is zero or very low, and provides no-volt and overload protection.
A DC motor should not be connected directly to the supply at standstill, as its low armature resistance can cause a large inrush current. The 4-point starter solves this by adding series resistance at startup and removing it gradually as the motor speeds up.
It is useful for smooth start-up, controlled acceleration, and field-based speed control, combining current limiting and protection in a single arrangement.
Role of the 4-Point Starter in DC Motors
| Function of the 4-Point Starter | Practical Purpose |
|---|---|
| Limits starting current | Prevents excessive armature current at standstill |
| Allows gradual acceleration | Removes resistance step by step as speed rises |
| Provides no-volt protection | Returns the handle to OFF during supply failure |
| Provides overload protection | Trips the starter under unsafe current conditions |
Uses and Advantages of the 4-Point Starter
A 4-point starter is used with DC shunt motors and DC compound motors, in systems that use field weakening for speed control or allow operation above normal speed through field adjustment.
Its main advantage is evident when the field current is intentionally adjusted with a rheostat. In this condition, the 4-point starter keeps the no-volt coil on an independent path, so reducing field current does not weaken the holding force of the starter handle.
Because of this, the 4-point starter is often a better choice than a 3-point starter in applications that require stable operation during field-based speed control. It is used in teaching setups, legacy DC motor drives, and industrial systems that still use classical DC machine control methods.
Construction of the 4-Point Starter
The name 4-point starter comes from its four terminals and the fact that the no-volt coil is connected through its own separate path rather than in series with the shunt field winding. This separate connection is the construction feature of the 4-point starter.
In the circuit layout, the armature is connected through the starting resistance, which is gradually removed as the handle moves from OFF to ON. The shunt field winding is connected to the field terminal, while the no-volt coil is connected independently across the line through its own resistance path. This arrangement keeps the holding action stable during field-current adjustment.
Operation of the 4-Point Starter in Practice
Operating Sequence of the 4-Point Starter
• Handle starts at OFF
• Maximum starting resistance is inserted in series with the armature
• Motor begins to accelerate
• Back EMF rises with speed
• Armature current becomes controlled
• Resistance is removed progressively
• No-volt coil holds the handle in the ON position
• Motor continues normal operation with starting resistance fully cut out
No-Volt Coil Action in the 4-Point Starter
The no-volt coil keeps the handle in the ON position against spring force. In a 4-point starter, this holding action is independent of field-current changes, which makes it more suitable than a 3-point starter for field-weakening applications.
Protection and Limits of the 4-Point Starter
Protection Actions
In a 4-point starter, loss of supply voltage causes the no-volt coil to lose its holding force, allowing the spring to return the handle to the OFF position. During excessive current, the overload release coil operates and disconnects the motor from the supply.
Main Limitation
The limitation of the 4-point starter is that it does not provide protection against field failure. If the field circuit opens, the flux may collapse to a very low value, leaving only residual magnetism. Since the speed of a DC motor is inversely proportional to flux, the motor may accelerate to a dangerously high speed. Additional field-failure protection is required in applications where overspeed could occur.
4-Point Starter vs 3-Point Starter
| Feature | 3-Point Starter | 4-Point Starter |
|---|---|---|
| No-volt coil connection | In series with the shunt field | Separate from shunt field |
| Response during field weakening | May trip unintentionally | More stable holding action |
| Suitability for speed control by field adjustment | Less suitable | More suitable |
| Field-failure protection | Some protection through the no-volt coil, but it may trip during field weakening | No inherent field-failure protection |
Conclusion
The 4-point starter is a basic starting and protection device for DC shunt and compound motors. It limits starting current, allows smoother acceleration, and provides no-volt and overload protection. Its separate no-volt coil connection makes it more suitable than a 3-point starter for field weakening. However, it does not protect against field failure, so extra protection may still be needed in some motor systems.
Frequently Asked Questions [FAQ]
Can a 4-point starter be used with a DC series motor?
No. A 4-point starter is used with DC shunt and compound motors, not DC series motors.
Why is the handle moved manually?
It is moved manually to reduce the starting resistance and safely control the motor during startup.
What happens if the starting resistance is removed too quickly?
The motor may draw excessive current, causing the armature to overheat and damage the starter contacts.
Does a 4-point starter improve efficiency during normal running?
No. Its main purpose is safe starting and protection, not improving normal running efficiency.
What signs show that a 4-point starter needs maintenance?
Common signs include rough handle movement, overheating, burnt contacts, failure to stay in the ON position, and frequent tripping.
Why is a 4-point starter less common today?
It is less common because modern electronic motor controllers provide smoother starting, better control, and improved protection.
